The present invention relates to a gear mechanism comprising inner gear teeth formed at a first gear element and outer gear teeth formed at a second gear element and engaged with the inner gear teeth, and a reduction planetary gear comprising a sun gear, a planetary gear mechanism driveably engaged with the sun gear, a ring gear including inner gear teeth formed inside thereof and driveably engaged with the planetary gear mechanism, and a case rotatably supporting the ring gear.
Conventionally, such gear mechanism comprising inner gear teeth formed at the first gear element and outer gear teeth formed at the second gear element and engaged with the inner gear teeth is known (see, e. g., Japanese Patent Laid-Open Publication Nos. 10-246173, 11-22789, and 2002-97668).
A gear mechanism disclosed in the Japanese Patent Laid-Open Publication No. 10-246173 is a multiplying planetary gear used for a windmill or the like, which transfers a rotation of a ring gear conveyed from an input shaft to an output shaft with speed multiplying. A gear mechanism disclosed in the Japanese Patent Laid-Open Publication No. 11-22789 is a reduction planetary gear used for a running drive device of a construction machine or the like, which transfers a rotation of a hydraulic pressure motor to a housing as a driven rotational member with speed reducing and drives the housing. A gear mechanism disclosed in the Japanese Patent Laid-Open Publication No. 2002-97668 is a turning device for the construction machine, which transfers a rotation of the hydraulic pressure motor provided at an upper end thereof to an output shaft with speed reducing via a reduction planetary gear, thereby turning an upper part with respect to an lower running part thereof. These gear mechanisms disclosed in the above publications comprise inner gear teeth formed at the first gear element and outer gear teeth formed at the second gear element and engaged with the inner gear teeth, and transfer the input rotation with speed multiplying or reducing.
Herein, in the gear mechanisms disclosed in the above publications, the outer gear teeth tend to be extremely less durable compared to the inner gear teeth in the case where they are formed of the same material. That is, since the inner gear teeth located outside have a larger tooth thickness of its gear tooth base than the outer gear teeth and the outer gear teeth located inside are engaged more often than the inner gear teeth because the gear element with the outer gear teeth has a smaller diameter than that with the inner gear teeth, the outer gear teeth tend to be extremely less durable compared to the inner gear teeth in the case where they are formed of the same material. Thus, when the transfer load between the inner and outer gear teeth increases, a contact face pressure and a bending stress which occur accordingly increase as well. As a result, there occurs a large difference of durability between the inner and outer gear teeth, and thereby the durability of the gear mechanism is limited eventually by the outer gear teeth with less durability. Herein, in the event that larger contact face pressure and bending stress occur to the inner and outer gear teeth, increasing hardness of the outer gear teeth might be considered in order to improve the durability thereof. However, it may result in a further deterioration of the durability of the inner gear teeth, and thereby increasing hardness of the inner gear teeth will be needed contrarily.
Thus, it is needed to provide the gear mechanism which can reduce the contact face pressure and the bending stress occurring to the inner and outer gear teeth, improve the limitation of the durability of the gear mechanism due to the large difference of durability between the inner and outer gear teeth, and lessen the necessity of increasing hardness of the inner gear teeth.
Meanwhile, the reduction planetary gear, which comprises the sun gear, the planetary gear mechanism driveably engaged with the sun gear, the ring gear including inner gear teeth formed inside thereof and driveably engaged with the planetary gear mechanism, and the case rotatably supporting the ring gear, is known (see, e. g., Japanese Patent Laid-Open Publication No. 11-22789, and Japanese Utility-Model Laid-Open Publication No. 5-42812). A reduction planetary gear disclosed in the Japanese Patent Laid-Open Publication No. 11-22789 is configured of a two-stage planetary gear mechanism. The final (second) stage planetary gears of the planetary gear mechanism comprise a carrier including three thick ribs which are fixed to a housing (case) coupled to a motor via bolts at portions in a circumferential direction at regular intervals. Three planetary gears driveably engaged with inner gear teeth formed inside of the ring gear are rotatably supported by the carrier at portions in the circumferential direction at regular intervals. Further, a reduction planetary gear disclosed in the Japanese Utility-Model Laid-Open Publication No. 5-42812 is configured of a three-stage planetary gear mechanism, in which the final (third) stage planetary gears engaged with the ring gear are configured such that four thereof are disposed in the circumferential direction at regular intervals.
Herein, however, the reduction planetary gear disclosed in the Japanese Patent Laid-Open Publication No. 11-22789 has a problem that the contact face pressure and the bending stress which occur to gear teeth of the planetary gears may become too large because the number of the final planetary gears which are engaged with the inner gear teeth of the ring gear is just three, which may be a so small number. Herein, it may be considered to make a tooth width of the planetary gears and the inner gear teeth wider enough to maintain a sufficient strength. However, that may cause another problem that the entire length of the reduction planetary gear becomes improperly long, which should be inappropriate in terms of a layout-space utility of the reduction planetary gear. Meanwhile, the reduction planetary gear with four planetary gears of the final stage one disclosed in the Japanese Utility-Model Laid-Open Publication No. 5-42812 could solve the above-described problem, but may cause other problems that it has, for example, too many parts number and an improper complexity in structure of the reduction planetary gear.
Thus, it is needed to provide the reduction planetary gear which can reduce the contact face pressure and the bending stress occurring to gear teeth of the ring gear and the planetary gears, maintain properly a sufficient strength of planetary gears driveably engaged with the ring gear, and simplify the structure thereof.
Further, it is also needed to provide the reduction planetary gear which can reduce the contact face pressure and the bending stress occurring to inner gear teeth of the ring gear, maintain properly a sufficient strength of the inner gear teeth of the ring gear, avoid any improper treatments for hardening the inner gear teeth of the ring gear, and reduce any limitations in work loads for manufacturing and in designing structures.